Blood glucose meters (measuring devices of a component in the blood) for carrying out measurement of a blood glucose level are known. The blood glucose meter is one wherein a blood glucose level is quantitatively determined by optically measuring (color measuring) a degree of a color change of a test paper that is colored depending on the amount of glucose (grape sugar) in the blood. The color measurement of the test paper is carried out at a photometric unit provided with a light emission element and a light receiving element wherein light is irradiated on the test paper to measure an intensity of reflected light.
With such a blood glucose meter, after operations of supplying the blood (sample) to a test paper and developing thereat, the measurement of a blood glucose level is started, and this presents a problem in that since a time required from the supply of the blood to the test paper till the color measurement is not constant, an measuring error may be caused thereby. To avoid this, there is a demand for development of an automatic blood glucose meter capable of continuously, automatically performing a series of operations covering from supply and development to a test paper to measurement.
On the other hand, for the test paper, there is known a porous membrane having such an arrangement that a reagent is carried on a sheet substrate constituted of a porous material capable of absorbing a sample therein. This type of test paper is low in transmissivity, i.e. developability, because the micro pores in the sheet substrate are so small in size as about 0.5 μm, with the problem that it takes a long time for the development of a sample. The long time required for the development of a sample in this way is inconvenient for such an automatic blood glucose meter as mentioned above.
Where an isotropic, porous membrane whose pore size is uniform throughout the membrane is used as a test paper, an object that should be filtered out in a sample is removed at a membrane surface, thus presenting the problem that it takes a long time for the development of the sample. Slow development of the sample does not allow the sample in a small amount to be percolated up to a measuring surface, with the attendant problem that a sample to be supplied increases in amount.
As to an isotropic porous membrane, unless the membrane is completely isotropic, developing rates of a sample at opposite sides differ from each other, and thus, it becomes necessary to deal with such a membrane as having front-back both sides. In this connection, however, it is usual that the front and back sides of the membrane are not visibly discriminated from each other, so that when a slight degree of anisotropy is caused in the membrane upon manufacturing an isotropic, porous membrane, a problem arises in that measuring accuracy is influenced.
For measures for solving such problems as set out hereinabove, the present applicant has proposed test papers (see Japanese Patent Laid-open No. Hei 11-183474) including (1) a test paper laminating a first porous layer carrying a reagent capable of giving a color by reaction with a specified component in a sample, and a second porous layer having a function of separating an object that should be filtered out from the sample by filtration, wherein the sample is supplied from a side of the first layer, (2) a test paper as recited in (1) above, wherein the first layer and the second layer are, respectively, hydrophilic, (3) a test paper as recited in (1) or (2) above, wherein micro pores in the first layer have a size of 8 to 50 μm, (4) a test paper as recited in any one of (1) to (3) above, wherein micro pores in the second layer have a size of not larger than 5 μm, and (5) a test paper as recited in any one of (1) to (4) above, wherein the sample is made of blood and the object that should be filtered out is blood cells mainly composed of blood red cells.
The present applicant has also proposed a test paper (see Japanese Patent Laid-open No. 2001-164030) made of a porous membrane which has an average pore size ranging 0.1 to 2 μm, a thickness of 5 to 200 μm and a porosity of 50 to 95% and which is anisotropic in such a way that a ratio between average pore sizes in one surface and the other surface is 1.5 or over.
However, where the test paper set out in the Japanese Patent Laid-open No. Hei 11-183474 is used, the lamination step of the first and second layers are necessary, thus making the manufacturing process complicated. Moreover, where the test paper described in the Japanese Patent Laid-open No. 2001-164030 is used, it is required to rapidly develop a blood plasma component reacted with a reagent toward a measuring surface while filtering out blood cells, with the following problems being involved. More particularly, a smaller size is more effective for separation and removal of blood cells by filtration, and too small a pore size leads to slow development of a blood plasma component. In addition, when blood cells are removed while keeping a given developing rate by making a large pore size at an inlet of the sample and a small pore size at an outlet thereof, hemoglobin in the blood cell component becomes visible through the porous structure if the blood cell removal is carried out just beneath the measuring surface, thereby giving an influence on measurement accuracy. In addition, when portions having pores of large size are large in area, a surface area is lessened, thus having one unable to sufficiently carry a reagent necessary for the measurement.